What affects the linear mass density of a string?

I had this question : if a diameter of a string is increased , how will the linear mass density(u) value changes ?
the answer was it will increase because mass will increase , but how the mass has increased ?

Is it not just as simple as: larger diameter -> more material per unit length -> more mass per unit length? Some other factors that affect the linear density could be the material used, temperature of the string, whether the string is being stretched.

I'm not sure whether your question refers to increasing the diameter by (somehow) adding more material inside the string, or to increasing the diameter from the outside.